Effects of uric acid dysregulation on the kidney

Abstract

Recently, research has redirected its interests in uric acid (UA) from gout, an inflammatory disease in joints, to groups of closely interrelated pathologies associated with cardiovascular and kidney dysfunction. Many epidemiological, clinical, and experimental studies have shown that UA may play a role in the pathophysiology of the cardiorenal syndrome continuum; however, it is still unclear if it is a risk factor or a causal role. Hyperuricemia has been well studied in the past two decades, revealing mechanistic insights into UA homeostasis. Likewise, some epidemiological and experimental evidence suggests that hypouricemia can lead to cardiorenal pathologies. The goal of this review is to highlight why studying both hyperuricemia and hypouricemia is warranted as well as to summarize the relevance of UA to kidney function.

INTRODUCTION

Uric acid (UA) was historically known as a waste product of purine breakdown, with its foremost clinical relevance being gout. Nevertheless, research in the last few decades has shown that both increased and decreased levels of serum UA (hyperuricemia and hypouricemia, respectively) are associated with a plethora of chronic conditions including chronic kidney disease (CKD) (4, 12, 14, 24, 31). Hyperuricemia, in particular, has been linked to cardiovascular disease (CVD) and CKD in both hypertension (HTN) and normotensive populations (18, 40, 44). Clinically more frequent of the two conditions is hyperuricemia (~20% prevalence; Ref. 49). It may be associated with impaired renal UA homeostasis triggered by the reduction in glomerular filtration rate (GFR), diuretic usage, increased renal vascular resistance, and coexistent insulin resistance in diabetic nephropathy (25, 30). The opposite condition, hypouricemia, although less common (prevalence 0.2–0.58% in the general population; Ref. 29), leads to nephrolithiasis and may occur as a result of overtreatment of hyperuricemia, defective renal tubular reabsorption of UA, and genetic mutation affecting purine metabolism (37).

The xanthine dehydrogenase (XDH) gene encodes for the enzyme group responsible for converting xanthine and hypoxanthine to UA, which is the end product of purine catabolism in humans. This enzyme group is known as xanthine oxidoreductase (XOR) and includes both XDH and xanthine oxidase (XO). Clinical (3, 36) and animal (6, 35, 39) studies have shown that reducing UA by inhibiting XOR blunts the progression of HTN, CVD, and CKD. In addition, recent clinical study-based perspectives suggest that UA-lowering therapy could be potentially used in hyperuricemic CKD patients without gout or nephrolithiasis (33).

Both current and impending UA-lowering therapies necessitate investigation into the effects of hypouricemia as the reduction of UA below the normal level with therapeutic management poses additional pathogenic risks. There are large-cohort studies to support the idea that both increased and decreased levels of UA are pathogenic (15, 17, 40, 44). In one such study, levels of circulating UA exhibited a J-shaped association with all-cause mortality of CKD, showing the lowest risk in the middle quintiles (40). Additionally, a prospective cohort study in healthy (based on GFR) people proposed that hypouricemia is a candidate predictor of kidney function decline (15). This indicates that not only hyperuricemia but also hypouricemia may impact CVD, renal function, and consequent mortalities.

UA EVOLUTION AND GENERATION

Humans have higher UA levels (3.5–7.0 and 2.6–5.7 mg/dL for men and women, respectively; Ref. 4) compared with nonprimate mammals (0.5–1.5 mg/dL; Ref. 46) due to an evolutionary mutation they have in the gene encoding uricase enzyme (UOX). In rodents, this enzyme further converts UA into allantoin (47). There are different speculations as to why this mutation persisted in humans. One theory states that increased UA provided an evolutionary advantage as a potent antioxidant (8) and may compensate for the loss of the ability to produce endogenous ascorbic acid (13, 46). UA acts as an antioxidant by scavenging carbon-centered radicals and peroxynitrite in the extracellular hydrophilic environment (4). Another hypothesis is that UA was needed to regulate blood pressure and prevent hypotension at a time of low-salt consumption (46). This hypothesis further highlights the potential importance of UA for CVD and HTN.

Nucleotides can be synthesized via a de novo process or by recycling of free purine and pyrimidine bases through the salvage pathway. The salvage pathway helps conserve energy by reusing the breakdown products to produce nucleotide precursors (22). Changes in the enzymes involved in this pathway cause diseases associated with purine metabolism and/or UA homeostasis. In purine metabolism, UA is produced in the liver from xanthine and hypoxanthine catalyzed by XOR. Purines, such as xanthine, are derived from the degradation of endogenous nucleic acids and dietary sources of purines, such as red meat. Both XDH and XO participate in this pathway, but they require different coenzymes for catalysis and, therefore, result in different byproducts. XDH uses NAD⁺ and produces NADH, while XO uses H₂O₂ and produces superoxide radicals (Fig. 1A). XDH can be converted into XO reversibly through oxidation of the sulfhydryl group or irreversibly by proteolytic conversions under various conditions, including tissue ischemia (31).

Fig. 1.

Uric acid (UA) metabolism and its effects. A: the xanthine oxidoreductase (XOR) enzyme group includes both xanthine dehydrogenase (XDH) and xanthine oxidase (XO). This enzyme group is responsible for converting hypoxanthine and xanthine to UA, which is the end product of purine catabolism in humans. B: UA homeostasis. Production and breakdown of UA takes place in the liver and excretion occurs in the gastrointestinal (GI) tract and kidney. C: proposed mechanisms by which deviations in UA homeostasis affects renal tissues. *Other factors leading to hyperuricemia include kidney dysfunction causing decreased excretion, alcohol consumption, and endocrine dysfunctions. Several other reasons that give rise to hypouricemia consist of renal reabsorption problems, iron and molybdenum deficiencies, high estrogen levels, and some uricosuric drugs. vSMc, vascular smooth muscle cell; ROS, reactive oxygen species; RAAS, renin-angiotensin-aldosterone system.

UA HOMEOSTASIS

UA homeostasis refers to the relatively steady state of UA levels in the body, determined by the balance between the production and excretion of UA. In humans, one-third of UA excretion occurs in the gastrointestinal tract and two-thirds occurs in the kidney (Fig. 1B), where it is freely filtered. Of the filtered UA load, only ~10% is excreted. Reabsorption and secretion happen in the proximal tubule in a species-specific manner. In humans and rats, UA reabsorption is predominant compared with pigs and rabbits, where secretion of UA plays a major role (1, 21). Two key transporters important for UA reabsorption are urate transporter 1 (URAT1), encoded by the SLC22A12 gene, and glucose transporter 9 (GLUT9), encoded by the SLC2A9 gene. URAT1 and GLUT9 have been identified as the main transporter in urate handling on the apical and basolateral sides of the proximal tubule epithelium, respectively (7, 45). URAT1 has been shown to be regulated by α-protein kinase 1 (16), insulin (42), and glucocorticoid receptors (19). Recent large-scale whole exome sequencing association studies of serum urate and kidney function identified functional variants in both SLC22A12 and SLC2A9 genes as influencing serum urate levels (41).

HYPERURICEMIA

Hyperuricemia results from the overproduction or underexcretion of UA, causing increased circulating levels, which can exceed saturation limits and lead to deposits in the blood, joints, tissues, and urine. Traditionally, hyperuricemia’s major clinical relevance is gout. Gout is an inflammatory disease caused by excess UA deposition and is a common finding in patients with CKD (48), often presented with cardiovascular comorbidities. Clinical treatments for gout include lowering UA levels. As an example, the XOR inhibitor allopurinol (an isomer of hypoxanthine) is a Food and Drug Administration-approved drug commonly used in such cases. UA nephrolithiasis or stone formation can also present with or without gout as part of a metabolic disorder. A recent clinical study concluded that higher acid load to the kidney resulting in net urinary excretion is an important factor of idiopathic UA nephrolithiasis (2).

Hyperuricemia as a Disease Marker and a Causal Factor

More recent research shows that hyperuricemia is associated with pathologies other than gout. Hindering the production of UA by blocking XOR should have a protective effect against CVD and CKD. This has been tested yielding positive results (9, 32, 39). For example, inhibition of XOR with a nonpurine inhibitor, febuxostat, in spontaneously hypertensive rats improved HTN and endothelial dysfunction (39). Interestingly, a recent randomized clinical trial in healthy young men showed that the decrease of UA level should be severe and accompanied by nitric oxide (NO) synthase (NOS) inhibition (5) to observe changes in the endothelial function. However, this study was done in healthy individuals. Therefore, the interpretation might be different for older patients with previously diagnosed vascular conditions (5). Furthermore, it was reported that UA-lowering allopurinol, tested in a randomized trial in adolescents with essential HTN, attenuated their blood pressure to normotensive levels (9).

Groups with high CVD risk, including men, obese individuals, postmenopausal women, and patients with HTN, exhibit increased UA levels, resulting from a variety of mechanisms (14). Even though gastrointestinal excretion of UA is significant, decreased GFR can be a mechanism of changes in uricemia (43). Increased net tubular reabsorption and increased production of UA are other major factors affecting UA levels. In addition to being a marker of disease, there is evidence to suggest that UA can lead to CVD and CKD. Thus, in vitro studies revealed that UA stimulates cell proliferation in vascular smooth muscle cells (28). Vascular smooth muscle cell proliferation leads to atherosclerosis, a primary cause of many CVD. UA is also known to be proinflammatory (12); therefore, its increase could lead to HTN and endothelial dysfunction. In addition, UA can act as a potent antioxidant in the extracellular environment (1, 15, 17). It is thought that hyperuricemia in CVD might be a compensatory mechanism to counteract oxidative damage caused by reactive oxygen species (ROS). However, UA can also be considered a prooxidant, because ROS are made as byproducts during intracellular UA synthesis.

Previous research has elucidated a mechanism of CKD caused by hyperuricemia leading to UA crystal deposition in renal tubules (38). The models used for these studies have a severe and rapid increase in serum UA, therefore precluding studies of disease pathogenesis before crystal development. To study the potential pathogenic role of UA without crystal deposition, Mazzali et al. (23) conducted experiments using a rat model of mild hyperuricemia. The previous rat model of hyperuricemia was supplemented with both oxonic acid, a uricase inhibitor and UA itself (38), to produce a seven- to eightfold increase in UA. In contrast, the model used by Mazzali and colleagues was supplemented with only 2% oxonic acid and exhibited only a 1.5-fold increase in serum UA (23). Renal injury and fibrosis occurred in parallel with the development of HTN in mildly hyperuricemic rats but not normouricemic rats (23). These observations were accompanied by increased juxtaglomerular renin levels and decreased expression of NOS in the macula densa (23), which are known to result in afferent and efferent arteriolar vasoconstriction. Treatment of hyperuricemic rats with enalapril, an angiotensin-converting enzyme inhibitor, prevented renal damage and HTN (23). A similar effect was achieved using l-arginine to stimulate NO synthesis. This study suggests a hyperuricemia-renin-vasoconstriction mechanism of UA pathogenesis in the kidney. This mechanism is supported by human studies, which have also indicated renin activation in the presence of hyperuricemia (32). Collectively, these studies demonstrated that increased serum UA is pathogenic and contributes to the development of CKD, potentially through stimulation of the renin-angiotensin system and inhibition of NO synthesis. This process appears to be independent of UA crystal deposition.

The genetic differences in expression of uricase enzyme between rodents and humans pose a challenge in using rodent models in studying related pathophysiology. To avoid this interference from uricase enzyme and develop a spontaneously hyperuricemic model that more effectively recapitulate human disease, a knockout was created in C57BL/6J mice (Uox^−/−) (20). Compared with wild-type mice, these hyperuricemic mice had significantly elevated blood urea nitrogen alongside kidney damage. They demonstrated renal lesions followed by dilated Bowman’s capsules and tubules. This model exhibited apparent inflammation and immune cell infiltration as well as elevation in mRNA transcripts for inflammatory cytokines in the kidneys. These results suggest a mechanism of adverse renal outcomes in hyperuricemia through an inflammatory pathway.

Shown on Fig. 1C are a few examples of proposed mechanisms by which deviations in UA homeostasis affects renal tissues. However, more research is needed to validate these mechanisms and to explore additional mechanisms by which hyperuricemia may contribute to CKD progression.

HYPOURICEMIA

Hypouricemia, a decrease in serum UA (below 2 mg/dL in humans), leads to xanthinuria and, ultimately, to xanthine and hypoxanthine crystal deposition in the joints, muscles, and/or kidney. It can cause nephrolithiasis/urolithiasis with secondary renal damage and pain if deposited in muscles (30). Hypouricemia can result from inherited or transient causes (Fig. 1C). Mutations in genes encoding urate transporters (2, 20) or liver enzymes [XDH (Ref. 18), molybdenum cofactor sulfurase (MOCOS; Ref. 10), purine nucleoside phosphorylase (PNP; Ref. 21), and phosphoribosyl pyrophosphate (PRPP)] have been identified in patients presenting hereditary hypouricemia. Hypouricemia and renal tubule dysfunctions associated with mutations in these genes have been reported in many studies (2, 4, 8). Liver failure hindering the production of UA, high levels of estrogen (estrogen is a uricosuric agent; Ref. 43), and lack of micronutrients used as cofactors with XOR complex are among other causes of hypouricemia.

Associations Between Hypouricemia and Kidney Disease

Although research exploring hypouricemia is sparse, a mutation in the Xdh gene has been revealed to cause renal failure and early death in a knockout mouse model (Xdh^−/− mouse) (27). The homozygous animals in this model were smaller in body size compared with their wild-type littermates (27). Studies in Xdh^−/− mice have shown that the renal damage is due to increased ROS production and the resulting interstitial fibrosis (26). The renal damage was accompanied by crystal and lipid depositions (26). These mice also exhibited an increased excretion of hypoxanthine and xanthine in their urine (26). These results, in combination with increased blood urea nitrogen (27), showed that Xdh^−/− mice most likely die due to renal failure as the underlying cause. A significant increase in markers for fibrosis, inflammation, ischemia, and oxidative stress was also seen in protein and mRNA levels in hypouricemic mice (26). One of the markers tested was transforming growth factor-β (TGF-β). A recent study done on hepatocellular carcinoma cells revealed that the downregulation of XDH can promote the TGF-β signaling pathway (5). TGF-β can induce cyclooxygenase-2 expression (9), which can ultimately produce prostaglandins promoting inflammation. Even though in humans hypouricemia is mostly a rare and asymptomatic condition, animal and cellular study evidence points to a potential mechanism of hypouricemia leading to kidney diseases through inflammatory signaling pathways.

Hereditary Diseases of Hypouricemia

Hereditary xanthinuria (HX) is an autosomal recessive disease characterized by low UA and elevated xanthine in the urine caused by mutations relating to xanthine production. There are two mutation-specific forms of HX. Type I is caused by a mutation in the XDH gene, and type II is due to a mutation in the MOCOS gene. There is a nonclassical clinically distinct third type that usually associated with severe neurological diseases (11) in addition to the symptoms of hypouricemia mentioned above.

In contrast with hypouricemic mice (27), both types of HX in humans are usually clinically mild or asymptomatic (31). The difference is likely due to the evolutionary pathway variance between mice and humans. Also, patients with HX have enhanced salvage pathway activity, recycling hypoxanthine into purines efficiently (22). This suggests that there is a compensatory mechanism to account for xanthine accumulation in patients with HX. Finally, during fetal development, humans can derive XDH from their mother, which is not possible in mice (26).

Genetic diseases of UA overexcretion (hyperuricosuria) are called renal hypouricemia. Like HX, they have been categorized into two types. The mutation causing types 1 and 2 are in genes SLC22A12 and SLC2A9, respectively (29). Renal hypouricemia can be asymptomatic until the patients are subjected to strenuous exercise, which can lead to acute renal injury (10, 34). Therefore, identifying these mutations is critical to avoid such episodes. Research indicates that this occurs due to oxidative damage caused by increased ROS production during exercise leading to renal vasoconstriction and ischemia (28).

CONCLUSIONS

Recently, many studies have elucidated the relationship between UA, CVD, and CKD, among other chronic conditions. Because of its common occurrence with and without gout, hyperuricemia was studied more thoroughly in research. This review focused on describing facets of hyperuricemia and hypouricemia linked with an increased risk for cardiovascular events as pathological states mediated by the disturbance in UA homeostasis and renal dysfunction. Given the sheer amount of evidence, it is apparent that serum UA levels should be maintained to protect the kidneys and prevent cardiovascular risks. Comprehensive studies are necessary to identify how UA homeostasis in the kidney is a pivotal mechanistic player in cardiovascular and renal health.

GRANTS

This work was partially supported by National Heart, Lung, and Blood Institute Grants R35-HL-135749 and P01-HL-116264 and Department of Veteran Affairs Grant I01-BX-004024.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

L.V.D. prepared figure; L.V.D. and A.S. drafted manuscript; L.V.D., D.R.S., O.P., and A.S. edited and revised manuscript; L.V.D., D.R.S., O.P., and A.S. approved final version of manuscript.